1. Field of the Invention
The invention concerns a procedure for the production of oxalic acid diesters by the oxidative carbonylization of alcohols with the use of palladium catalysts.
2. Background of the Invention
The production of oxalic acid esters from the apropriate alcohols by means of oxidative carbonylation through the catalytic effect of a metal from the platinum group has been described many times.
From U.S. Pat. No. 3,393,136 a procedure for the production of saturated oxalic acid esters is known, in which an essentially water-free reaction medium, consisting of a saturated simple C.sub.1 -C.sub.12 alcohol and 0.001 to 2% by weight of a metal from the platinum group as well as 0.05 to 5% by weight of a redox salt from the group of soluble Cu(II) and iron (III) salts, is brought into contact with carbon monoxide and oxygen simultaneously, in order to retain the redox salt at its highest oxidation level, at temperatures between 30.degree. and 300.degree. C. and pressures between 5 and 700 atmospheres. In DE-OS No. 22 13 435 there is also a suggestion for the production of oxalic acids and their esters in which carbon monoxide in an aqueous or alcoholic phase is oxidized with oxygen under pressure and at an elevated temperature in the presence of a catalytic system composed of a salt or a complex of a platinum metal and a salt or a complex of another metal which is more electropositive than the cited precious metals and which can appear at various oxidation levels. Also required is the conversion of insoluble or difficultly soluble platinum metal salts into soluble complexes by the addition of an alkali salt.
DE-OS No. 25 14 685 describes a procedure for the production of dialkyl oxalates by the conversion of an aliphatic alcohol with carbon monoxide and oxygen under pressure in the presence of a catalyst (consisting of a mixture of a salt, a metal of the platinum group and a salt of copper or iron) and possibly in the presence of an alkali metal salt, whereby the conversion is carried out in the presence of an activator. The group of cited "activators" is--in chemical terms--very inhomogenous. It includes, e.g., inorganic bases such as alkali hydroxides and carbonates as well as neutral substances known for their drying effect, such as, e.g., Na.sub.2 SO.sub.4 and MgSO.sub.4. The effect of nitrates may be related to the participation of nitrites or esters formed in the redox procedures during the oxidative carbonylization [see also European application No. 0 056 993, European application No. 0 056 994, DE-OS No. 27 33 730, European application No. 0 057 629, European application No. 0 057 630, European application No. 0 046 983, Japanese published application No. 83-21646, (Chem. Abstr. 98, 160258a), Japanese published application No. 83-126836 (Chem. Abstr. 99, 194450j), and European published application No. 0 105 480].
From DE-OS No. 26 01 139 a procedure is known which is characterized by the use of added ammonia or amines. A further development is suggested in DE-OS No. 27 21 734, according to which a halide-free ammonium salt is used in addition to amines and the copper (II) compound is also supposed to be free of halide ions.
In U.S. Pat. No. 4,005,128 the use of at least stoichiometric amounts of amines under the exclusion of halides of the catalyst metals is suggested. According to U.S. Pat. No. 4,005,129 and DE-PS No. 27 21 734 this suggestion is supplemented by the addition of halogen-free ammonium salts, e.g. salts of sulfuric, acetic or trifluoroacetic acid. In DE-OS No. 28 14 708 the use of acidic co-catalysts is recommended.
Although the general suitability of precious metal catalysts of the platinum group in conjunction with redox systems such as Cu-II or Fe-III for the catalytic effect on the oxidative carbonylization of alcohols could be deduced from the state of the art, the various proposed variations presented a rather confusing, if not contradictory, picture.
More recently, heterogeneous catalysis with palladium containing mixed catalysts has gained increasing attention. In U.S. Pat. No. 4,447,638, the catalytic effect of a mixed catalyst of Pd or Pd salts in combinaton with crystalline vanadium oxide, phosphorus oides and titanium oxides has been employed. In U.S. Pat. No. 4,451,666 a comparable catalyst system is recommended, in which titanium has been replaced by manganese. According to U.S. Pat. No. 4,447,639, iron instead of titanium or manganese is used in the mixed catalysts. From Japanese published application No. 84,05 143 (Chem. Abstr. 100, 174285v) a procedure is known in which the carbon monoxide and oxygen, together with catalysts that contain tetramine-palladium(II)-nitrate on carbon, is reacted in the autoclave, and in Japanese published application No. 84-05144 (Chem. Abstr. 100, 113511q) a similar catalyst, containing T1 in addition, is recommended. In Japanese published application No. 81-142239 (Chem. Abstr. 96,68378), a process is suggested in which palladium metal has been added to activated carbon and the halogen components are present as the soluble component of the catalyst.
As a rule, in homogenous catalysis a considerable reduction of the catalytic effect is observed during continuous operation. Heterogeneous contact catalysis can provide certain advantages as compared to homogenous catalysis. It is, for example, usually easier to separate the catalyst components. This makes their reuse simpler. In addition, it does not cause precipitation of the heavy metal ions in the solution, the recycling of which into the original catalyst form can be very expensive.
However, even the state-of-the-art procedures which operate with heterogeneous catalysis are not wholly satisfactory. Especially the long reaction times and the yield obtained leave much to be desired.
Thus, there remains the problem of developing heterogeneously working contact catalysts which, with relatively short reaction times, provide a high yield of oxalic acid esters and have a high selectivity in the oxidative carbonylation of alcohols. In addition, the catalyst system should not be less readily accessible and not more costly than the ones already in use.